The present invention relates to improvements in or relating to solid-state surface-emitting optical devices. In particular, the invention relates to surface-emitting optical devices having structures based on the InAlGaN quaternary system, especially short-wavelength (less than 600 nm) Gallium Nitride (GaN) vertical-cavity surface emitting lasers (VCSELs) and GaN surface emitting diodes.
According to a first aspect of the present invention there is provided a solid state, surface-emitting, optical device having a body of optical gain medium overlying a high reflectivity distributed Bragg reflector (DBR) mirror which is carried by an underlayer,
wherein the DBR mirror is a multi-layer dielectric fabrication having alternate layers of dielectric material with a high refractive index ratio between the adjacent layers in the fabrication, and the body of optical gain medium is part of an epitaxial layered structure extending from the underlayer and over the fabrication.
By virtue of the DBR mirror being formed of dielectric material, the high refractive index ratio can be greater than 1.2; preferably, is greater than 1.3; advantageously, is greater than 1.5, as a result of which few periods (preferably, less than fifteen periods; advantageously less than ten periods) are required to produce a highly reflective mirror (which, as is typical in laser devices, has a reflectivity of the order of 97% or greater) which has the advantage that the fabrication process is simple
Preferably, the fabrication is one of an array of columns having a lateral dimension of less than approximately 50 xcexcm and spaced part (from centre to centre) by less than approximately 100 xcexcm; advantageously, the columns have a lateral dimension of less than approximately 10 xcexcm and are laterally spaced by less than approximately 20 xcexcm. Alternatively the fabrication may be one of an array of stripes or lines extending to a length of 100 xcexcm or more, separated by a small number of xcexcm, typically about 10 xcexcm, and having a width comparable in dimension to the spacing.
It will be appreciated that the underlayer will usually be a substrate having a buffer layer; preferably, the substrate is sapphire, alternatively, the substrate is SiC; preferably, the buffer layer is based on any of the group three (periodic table) nitride. If high quality substrates are available then the underlayer may consist of only the substrate.
The underlayer is typically a plate-like component with the DBR mirror fabrication carried by one surface and with the epitaxial layered structure extending from that surface. The surface may be planar with the fabricated array of columns or stripes upstanding from the planar surface. Alternatively, the surface may be patterned to from columnar or striped depressions in which the fabricated mirror array is located. In each case the epitaxial layered structure extends from the surface and over the fabrication. In the limiting case the depressions extend through the thickness of the component and the DBR mirror fabrication is carried by both the component and the epitaxial layered structure.
The epitaxial structure is formed by combinations from the InAlGaN quaternary system, for example, GaN or alloys thereof. Preferably, the epitaxial structure includes an Indium Gallium Nitride-based (InGaN) Multi-quantum well structure. Such epitaxial structures are variously referred to as homo-epitaxial and hetero-epitaxial.
Preferably, one of the alternate layers in the multi-layer dielectric fabrication is silicon dioxide (SiO2) and the other alternate layer is titanium dioxide (TiO2). The SiO2/TiO2 combination has a very high refractive index ratio (approximately 1.8) and is particularly suitable for operation near the 450 nm wavelength where absorption is very low. Other suitable dielectric layers may be used, however, and these include: MgF2, CaF2, Al2O3, ZnS, AlN, SiC, Si3N4 and ZrO2; in combinations such as: SiO2/SiC, SiO2/Si3N4, CaF2/ZnS, Al3O3/TiO2, SiO2/AlN, and SiO2/ZrO2. The SiO2/ZrO2 combination is particularly suited to operation at about the 400 nm wavelength and has a refractive index radio of about 1.4.
Preferably, the body of optical gain medium is surmounted by a conductively-doped layer and overlies a conductively-doped layer surmounting the DBR mirror and electrodes are connected to the conductively-doped layers for electrical activation of the device, whereby the device is operably as a diode.
Preferably, a further mirror which is partially optically transmissive is disposed on the epitaxial structure in registration with the DBR mirror so that the epitaxial structure functions as a solid state optical cavity.
Where the optical device is a light-emitting diode, the further mirror has a reflectivity in the range from approximately 50% to 90%, so that lasing is not initiated. Where the optical device is a VCSEL, the further mirror has a reflectivity higher than approximately 98%, so that lasing is initiated and, provided that the underlayer is transmissive, the lasing output may be taken either through the DBR mirror or the further mirror according to the respective reflectivities.
The further mirror may be made of any convenient materials, such as semiconductors, metals and/or dielectrics.
According to a second aspect of the present invention there is provided a method of fabricating a solid-state, surface-emitting, optical device incorporating an improved distributed Bragg reflector (DBR) mirror, the method comprising the steps of:
providing an underlayer;
growing a multi-layer coating on the underlayer, the coating comprising alternate layers of high refractive index dielectric and low refractive index dielectric;
selectively removing portions of the coating to provide an array of free-standing dielectric fabrications whereby portions of the underlayer are revealed between adjacent fabrications;
expitaxially growing a semiconductor layered structure incorporating a body of optical gain medium on the revealed portions of the underlayer so that a lower part of the structure grows up and laterally on top of the free-standing dielectric fabrications, and an upper part of the structure incorporates the body of optical gain medium and overlies the fabrications so that one of the free-standing fabrications provides the DBR mirror.
By virtue of this aspect of the present invention, an efficient surface-emitting optical device (such as a GaN VCSEL) incorporating a DBR mirror having few periods may be fabricated. The optical gain medium overlying the DBR mirror is substantially defect-free because the mirror stops threading dislocations propagating from the underlayer. Because threading dislocations propagate vertically, the optical gain medium above the DBR is laterally offset from any threading dislocations propagating from the underlying layer.
The method may further comprise the steps of
growing a further mirror on the body of optical gain medium;
providing a first electrode electrically connected to one side of the optical gain medium in registration with said one of the free-standing fabrications; and
providing a second electrode electrically connected to the opposite side of the optical gain medium;
so that the optical gain medium functions as an optical cavity which may be electrically activated by the electrodes.
Conveniently the fabrication is in the form of an array of individual columns or of stripes (lines) extending parallel to the crystallographic direction  less than 1, xe2x88x921, 0, 0 greater than  of the underlayer.
Preferably the array of fabrications is provided by pattern etching. Alternatively the xe2x80x98lift offxe2x80x99 technique may be used whereby a pattern of photo-resist material is deposited prior to the multi-layer deposition coating and is subsequently chemically dissolved to remove the overlying multi-layer deposition and to leave the intervening areas of the multi-layer deposition which thereby form the column or striped fabrications.
According to a third aspect of the present invention there is provided a method of fabricating a solid-state surface-emitting optical device incorporating an improved distributed Bragg reflector (DBR) mirror, the method comprising the steps of:
providing an underlayer;
selectively patterning a surface of the underlayer to provide an array of depressions in the surface;
providing an array of dielectric fabrications in the depressions with portions of the underlayer revealed between adjacent fabrications, each fabrication comprising alternate layers of high refractive index dielectric and low refractive index dielectric; expitaxially growing a semiconductor layered structure incorporating a body of optical gain medium on the revealed portions of the underlayer so that a lower part of the structure grows up and laterally on top of the free-standing dielectric fabrications, and an upper part of the structure incorporates the body of optical gain medium and overlies the fabrications so that one of the free-standing fabrications provides the DBR mirror.
According to a fourth aspect of the present invention there is provided a method of fabricating a solid-state surface-emitting optical device incorporating an improved distributed Bragg-reflector (DBR) mirror, the method comprising the steps of:
providing an underlayer of gallium nitride;
patterning the underlayer with laser-drilled holes;
expitaxially growing a semi-conductor layered structure incorporating a body of optical gain medium on a surface of the underlayer so that the lower part of the structure grows up and laterally on the surface and overlies the holes therein; and
fabricating a multi-layer coating within the thickness of the holes so that the fabrications are carried by both the underlayer and the epitaxial layered structure overlying the holes.
By selecting the optical gain medium the optical device may operate at wavelengths less than approximately 1 xcexcm; in particular, by selecting an InGaN-based optical gain medium the optical device may operate at wavelengths less than 650 nm, with anticipated optimal performance at approximately 400-450 nm.